Dissipation of electrostatic charges from insulating materials

ABSTRACT

1. A METHOD OF MANUFACTURING AN ELECTRICALY ONDUCTIVE PLASTIC TUBE COMPRISING THE STEPS OF: MOLDING A PAIR OF PREFORM TUBE HALVES FROM AN EXTRUDABLE, SINTERABLE PLASTIC MATERIAL INCORPORATING A VOLATILE LUBRICANT, EACH PREFORM BEING A RELATIVELY THICK WALLED SEMI-CYLINDRICAL MEMBER HAVING A LONGITUDINAL SEMI-CYLINRICAL BORE, THE BORE OF ONE OF SAID PREFORMS HAVING A LARGER RADIUS THAN THE OTHER, APPLYING A CONDUCTIVE COATING COMPRISING EXTRUDABLE, SINTERABLE PLASTIC MATERIAL INCORPORATING FROM 3 TO 25% BY WEIGHT CONDUCTIVE FILLER AND VOLATILE LUBRICANT WITHIN THE BORE OF LARGER RADIUS TO SUCH A THICKNESS AS TO REDUCE THE BORE TO THE SAME SIZE AS THE BORE OF THE OTHER PREFORM, PLACING THE TWO PREFORMS IN FACE TO FACE RELATION TO DEFIN A CYLINDER AND EXTRUDING THE SAME THROUGH A DIE TO FORM A TUBE, DRIVING OFF SAID LUBRICANT, AND THEREAFTER SUBJECTING SAID TUBE TO ELEVATED TEMPERATURES TO EFFECT SINTERING OF SAID TUBE.

0d. 29, 1974 SHAW $845,186

DISSIPATIOH OF ELECTROSTATIC CHARGES FRO INSULATING MATERIALS OriginalFiled NOV. 25, 1970 ATTOR'A/[V United States Patent 3,845,186DISSIPATION OF ELECTROSTATIC CHARGES FROM INSULATING MATERIALS AllenFrederick Shaw, near Reading, Berkshire, England, irssignor toSuperflexit Limited, Buckinghamshire, Engand Continuation of applicationSer. No. 92,780, Nov. 25, 1970. This application June 23, 1972, Ser. No.265,480 Int. Cl. B29f 3/00 U.S. Cl. 264-104 1 Claim ABSTRACT OF THEDISCLOSURE The present invention relates to means for dissipating anelectrostatic charge from a tube formed of an insulating material byforming a strip of conducting material within the tube by extruding apreform of the plastics material having a conductive insert andsintering the extrusion at an elevated temperature.

This is a continuation of application Ser. No. 92,780, filed Nov. 25,1970 now abandoned.

The present invention relates to a method of providing a means ofdissipating an electrostatic charge from an insulating material and inone aspect of the invention provides means for dispersing or dissipatingelectrostatic charge from a conduit or tube of an insulating material.

Polytetrafluoroethylene, commonly known as P.T.F.E. and hereinafter soreferred to is extensively used in tube form for fuel lines and fuelhose lines and particularly in aircraft engine installations and thelike where extreme conditions of temperature and pressure areencountered during the engine operation and where a high degree ofsatisfactory performance and useful life is essential. Where suchreliable performance characteristics are required, the tubing ispreviously subjected to rigid inspection procedures and must meetcertain specifications and tests before acceptance for installation. Thesame is true of other insulating materials which can be used in a tubeor conduit for the passage of liquid therethrough. In the case ofP.T.F.E. tubing, the tube is conventionally covered with an armouredsheath in the form of a flexible braid covering. Although rigidlyinspected and tested equipment has given satisfactory performance, ithas been noted that over a long period of time premature failure orundue leakages have frequently occurred. During passage of fuel orliquid through P.T.F.E. tubes and tubes formed of other plasticsinsulating material, an electrostatic charge builds up in the lines. Inview of the nature of the operating conditions, this electrostaticcharge has not, as would be expected, been dissipated from the interiorof the tube to the end fittings. Rather it has appeared in certainfailures that static electricity has been discharged by arcing throughthe walls of the P.T.F.E. tube to the earth by way of the metal of thewire braid covering. This arcing results in the formation of a minutepin-hole in the wall structure and thus causes leakage of the fuel orother liquid passing through the tube.

Considerable problems have arisen in the grounding of electrostaticcharges which build up in conduits and containers in which liquids orgases are circulated and in some circumstances these charges can giverise to dangerous or metastable conditions.

According to the present invention there is provided a method forproviding means for dissipating an electrostatic charge from anextrusion of an insulating plastics material which method comprisesforming a preform of the said insulating plastics material having anaxial insert of conductive material comprising a conductor of fineparticle size and a matrix of plastics material the same as orcompatible with the said material, extruding the preform so formed toprovide an extrusion having a longitudinal strip of conductive materialin the surface thereof and sintering the extrusion at an elevatedtemperature.

In one aspect of the present invention, the preform is preferably formedin two parts at least one of which contains an insert of conductivematerial therein and the two parts of the preform are thereafterinserted in the extruder for extrusion in the normal way.

The present invention also provides a conduit, tube or container formedby extrusion from an insulating material having in an inner surfacethereof a longitudinal strip of conductive or substantially conductivematerial the arrangement being such that the strip serves to dissipatean electrostatic charge which may build up in said tube or containerduring use. A multiplicity of strips may be employed within said tube orcontainer to dissipate said electrostatic charge.

The plastics material may be polytetrafluoroethylene (P.T.F.E.) and theextrusion may be a tube formed in accordance with the process of thepresent invention and having at least one strip incorporated on an innersurface thereof.

The conductive material from which the conducting strip is formed maycomprise a dispersion of carbon black of fine particle size within amatrix of the polymer of the tube. In this way on sintering of thematerial the conducting strip becomes integral with and forms part ofthe tube itself. The matrix may be of the same material as the tube,i.e. polyethylene, polypropylene, polytetrafluoroethylene and the like,and may contain up to from 3 to 25% by weight of a conductive materialtypically carbon black or iron oxide (Fe 0 Any finely ground conductormay be employed in the conductive material or strip and in the case ofcarbon black this should be ground to a particle size of between 0.5 and20 mil. micron. The maximum amount of conductive material incorporatedwithin the matrix is that amount which can be incorporated in the matrixmaterial to permit the resulting mixture to retain its generallyplastics properties in that it is capable of being extruded andsubsequently sintered to form part of the extruded article itself.

The conductive material, as employed during the formation of thepreform, contains a proportion of a lubricant or solvent typically whitespirit or naphtha in a quantity of to by weight on the weight of theplastics material constituting the matrix of the conductive material.The lubricant selected is preferably the same as the lubricantincorporated in the insulating material during the extrusion process.

The preform is formed in a mold which is separated into two compartmentsby means of a pair of diametrically disposed separation plates carriedby a central cylindrical plunger. One side of the cylindrical plunger isprovided with a semi-circular brass sleeve approximately thick. Thematerial is applied to the mold on either side of the plunger andseparator plate in approximately equal quantities and it is, thereafter,pressed at a pressure within the range of to lbs. per square inch, tomold two preform elements. The preform elements are then removed fromthe mold and the first preform corresponding with the semi-circularbrass sleeve about-the centralplunger is provided with a layer ofapproximately ,4 thick of the conductive material formed in the methoddescribed above. The conductive material is pressed into place using thesame brass sleeve at hand pressure i.e. of the order of to 20 lbs. persquare inch. The two halves of the preform are inserted about thecentral pin of an extruder and the extrusion is carried out in aconventional manner using a typical reduction ratio of 300:1 to 400:1.-t f Following is a description by way of exampleonly and with referenceto the accompanying drawing of one method of carrying the invention intoeffect. j

In the drawing there is shown a perspective view partially in cut awayof a mold for formingthe preform inaccordance with the presentinvention.

An insulating mix forextruding a polytetrafiuoroethyrene tube was madeup of the following parts by weight:

Parts P.T.F.E. paste polymer 100 White spirit 20 Pigment cadmium red 0.5

The paste polymer, white spirit and pigment were thoroughly anduniformly mixed and thereafter the mixture was employed in the formationof a preform.

The preform is a two part preform and is formed in a mold 10 comprisinga hollow body 11 having a cylindrical bore 12 accommodating centralplunger 13 and pressure portion 14 disposed within said body. Thepressure portion closes one end of the cylindrical bore 12. The pressureportion is carried by a cylindrical plunger 15 disposed co-axial withinsaid cylindrical bore 12 and is adapted to be axially slidable togetherwith said pressure portion within the body in an axial direction toreject molded preforms from the mold. The plunger is provided with apair of diametrically disposed Wings 16 which extend longitudinally ofthe cylindrical bore from the pressure portion towards the other endthereof thereby forming a two compartment mold. A semi-circular brasssleeve 17 thick is placed about the central plunger between the wingsconstituting the mold partition to provide a spacing for a subsequentinsert of conductive material in one only of the two halves of thepreform.

The mixture prepared in the manner described above is thereafterinserted in equal quantities in each compartment, typically 310 grams ofmaterial being supplied to each side. The material is then pressed intoa preform by applying pressure from the top of the mold by means of asuitable plunger at a pressure of 150 lbs. per square inch. The pressureis released and the split preform section is driven out of the mold byraising the plunger and the base plate upwardly of the cylindricalsleeve, the two halves of the preform thereafter being separated fromthe wings, plunger and base plate to provide first and second preformportions, the former having been formed in the portion of the moldincorporating the semi-circular brass sleeve. The second portion isinserted in polymerand the first preform portion is coated within thespace defined by the insert the thick layer of a conductive mix. I

The conductive mix is formed of the following parts by weight:

Parts P.T.F.E. paste polymer 100 White spirit 100 Carbon black ground to0.5 micron 6 The material is coated into the recess at a thickness ofapproximately by hand and is thereafter pressed into place using asemi-circular brass sleeve. The pressure exerted as 10 to 20 lbs. persquare inch. After insertion of the conductive mix the first preformportion is inserted into the extruder over the central pin together withthe a 1 .4 second preform portion and is extruded in the usual way usinga reduction ratio of between 300:1 and400z1.

The extrudate was in the form of a tube having a A internal diameter anda wall thickness of 0.025". The following table illustrates theresistance properties against test result voltage for each sampleproduced by the above method, the resistance is given as a specificvolume resistance per 0.001 inch:

. j Test I voltage,

Sample number Length Resistance kv.

1 (control) No conductivity measurement recorded 1 500 volts.

Norm-Sample 1 (control) was made without a conductive strip.

These resistances were low enough to dissipate an electrostatic chargebuilt up in the tube.

In practice in the process of the invention two preforms were insertedin the press for extrusion through the die and they were arranged sothat the strips in each preform were aligned with the strips in theother. The tube so formed was thereafter cut at the joint and the tubewas dried for 24 hours to remove the lubricant at room temperature.Thereafter, the tubes were sintered at a temperature of 720 F. for 4hours.

In an alternative embodiment of the invention it will be appreciatedthat in a continuous extrusion process, the tube can be passed through adrying oven at a temperature of 320 F. to remove the lubricant andthereafter passed through a sintering oven at an elevated temperature of800 F.

I claim:

1. A method of manufacturing an electrically conductive plastic tubecomprising the steps of:

molding a pair of preform tube halves from an extrudable, sinterableplastic material incorporating a volatile lubricant, each preform beinga relatively thick walled semi-cylindrical member having a longitudinalsemi-cylindrical bore, the bore of one of said preforms having a largerradius than the other, applying a conductive coating comprisingextrudable, sinterable plastic material incorporating from 3 to 25% byweight conductive filler and volatile lubricant within the bore oflarger radius to such a thickness as to reduce the bore tothe same sizeas the bore of the other preform, placing the two preforms in face toface relation to define a cylinder and extruding the same through a dieto form a tube, driving off said lubricant, and thereafter subjectingsaid tube to elevated temperatures to effect sintering of said tube.

References Cited UNITED STATES PATENTS 2,414,793 l/ 1947 Becker et a1.264--104 3,354,243 11/1967 Dodge 264-25 3,442,715 5/1969 Yee et a1.264-104 2,945,265 7/1960 Sell et a1. 264176 R 3,473,087 10/1969 Slade138103 XR 3,070,132 12/1962 Sheridan 138103 XR 3,230,288 1/1966Henderson 264-176 R 3,152,082 10/1964 Davis et al 264-127 XR (Otherreferences on following p FOREIGN PATENTS Randolph et aL, Plasticsngineering Handbook, Reins31,7s4 3/1960 Great Britain 264-323 110,(1960) 501 relied 735,162 8/1955 Great Britain 264-426 ROBERT F. WHITE,Primary Examiner OTHER REFERENCES 5 W. E- HOAG, Assistant Examiner Remyet al., Treatise on Inorganic Chemistry, Elsevier, US Cl.

NY. (1956), p. 262 relied on.

Remy, H., Treatise on Inorganic Chemistry, Elsevier, 5 263; 317-2 1;N.Y. (1956), pp. 253 and 289 relied on. 10 68 C

1. A METHOD OF MANUFACTURING AN ELECTRICALY ONDUCTIVE PLASTIC TUBECOMPRISING THE STEPS OF: MOLDING A PAIR OF PREFORM TUBE HALVES FROM ANEXTRUDABLE, SINTERABLE PLASTIC MATERIAL INCORPORATING A VOLATILELUBRICANT, EACH PREFORM BEING A RELATIVELY THICK WALLED SEMI-CYLINDRICALMEMBER HAVING A LONGITUDINAL SEMI-CYLINRICAL BORE, THE BORE OF ONE OFSAID PREFORMS HAVING A LARGER RADIUS THAN THE OTHER, APPLYING ACONDUCTIVE COATING COMPRISING EXTRUDABLE, SINTERABLE PLASTIC MATERIALINCORPORATING FROM 3 TO 25% BY WEIGHT CONDUCTIVE FILLER AND VOLATILELUBRICANT WITHIN THE BORE OF LARGER RADIUS TO SUCH A THICKNESS AS TOREDUCE THE BORE TO THE SAME SIZE AS THE BORE OF THE OTHER PREFORM,PLACING THE TWO PREFORMS IN FACE TO FACE RELATION TO DEFIN A CYLINDERAND EXTRUDING THE SAME THROUGH A DIE TO FORM A TUBE, DRIVING OFF SAIDLUBRICANT, AND THEREAFTER SUBJECTING SAID TUBE TO ELEVATED TEMPERATURESTO EFFECT SINTERING OF SAID TUBE.